1. Field of the Invention
This invention is related generally to a method and apparatus for the separation and removal of ink from recycled fiber, and specifically to a method and apparatus which incorporates deinking, flotation, and bleaching and/or brightening operations, in a single stage by using a direct current (DC) electric field to induce electrophoresis and electrocoagulation to achieve better removal and separation of ink from the fibers, typically fiberized or pulped waste paper, and to provide better brightness and whiteness to the resulting pulp.
2. Prior Art
Deinking or ink removal is one of the key operations during fiber recycling, such as waste paper recycling. In a normal recycling operation, waste paper is fiberized in a hydrapulper in the presence of chemicals. The ink particles, dislodged or released from the fiber surfaces due to mechanical and/or chemical action, then are separated from the fiber slurry by means of a series of dispersion, flotation and/or washing steps, and other methods. To facilitate the complete removal of ink particles, more than one mechanical stage, including for example dispersion, flotation, washing, or other methods, typically are employed, or the ink particles are reduced mechanically or chemically to a smaller size such that they are not visible to the naked eye. Any other ink particles remaining in the pulp stock appear as specks in a paper sheet.
The several mechanical and chemical treatments incorporated in recycling processes further reduce the strength properties of fiber. To restore the fiber strength, additional chemical treatments typically are employed. All of these operations add to the cost of recycling and make recycling uneconomical compared to virgin paper making. Thus, there is a need for a technology that can accomplish deinking in a fewer number of mechanical and chemical stages so as to reduce fiber destruction and decrease cost, while maintaining the fiber properties at desirable levels. Therefore, more efficient means, preferably non-chemical and non-mechanical, are needed to reduce the number of repetitions and thereby retaining the strength properties of the fibers.
The use of an electric field to loosen foreign matter from and to cause the bleaching of fibers is known in the art. Brown, U.S. Pat. No. 366,557, discloses the use of an electric current to loosen any glutenous or any other foreign matter that may adhere to fibrous material such as ramie, pita, yucca, and other fibrous plants or materials for textile or other uses. However, the Brown process is geared toward the initial processing and cleaning of fibrous materials such that they may be further processed for textile or other uses, and does not disclose a method of removing ink and bleaching and/or brightening recycled fiber for reconstituting purposes. Ruthenberg, U.S. Pat. No. 747,234, discloses a method for bleaching paper pulp by applying an electric field across a paper pulp and chemical mixture. The Ruthenberg process adds a conductivity increasing chemical, such as calcium oxide, and uses the electrolytic decomposition of a chemical, such as sodium chloride, to create a bleaching agent. However, the Ruthenberg process is typical of electrolytic bleaching process well-known in the art today, and has the disadvantage of requiring the use of chemicals, which adversely affect fiber strength.
The use of an electric field to remove ink from a paper pulp slurry also has been disclosed in the prior art. Bonser, U.S. Pat. No. 1,008,779, discloses a process for removing ink from a paper pulp slurry by subjecting the paper pulp slurry to electrolysis. In the Bonser process, the paper is washed and pulped and then mascerated in the presence of a chemical preparation. The pulp is boiled and treated with an acid in water, drained, and then neutralized with an alkaline solution. The neutralized pulp slurry is then subjected to electrolysis. However, the Bonser process uses a common configuration for the electrolysis, both as to how the electric field is applied and the stage during the pulp processing at which the electric field is applied. Additionally, the Bonser process is carried out in a vat-type setup and does not produce paper pulp clean enough or bright enough for many applications.
Krodel et al., U.S. Pat. No. 2,743,178, discloses a process for deinking printed waste paper slurries, particularly low quality paper stock, such as waste newspaper stock, using an electric field. In the Krodel et al. process, the waste paper is shredded and combined with water and a suitable detergent to loosen the ink binder and emulsify the ink particles. A salt is added to charge the ink particles so as to partially separate them from the cellulose. The pulp then is heated and agitated to bring about a greater separation of the ink particles from the cellulose. Finally, an electric field is directed through the suspension to increase the separation of the ink particles from the cellulose. The Krodel et al. process emphasizes the addition to the pulp suspension of a certain type of salt to magnify the differences in electric charges between the fibers and the ink particles, and therefore has the disadvantage of unwanted chemical steps. The design of the Krodel et al. deinking vessel does not include any means for cleaning the electrodes, or for removing the ink from the vessel. Further, the Krodel et al. structure does not allow for the Production of a pulp which is clean enough or bright enough for many applications. In order to achieve such desired cleanness and brightness, the mere application of an electric current through a pulp is not sufficient.
Fujita et al., U.S. Pat. No. 3,835,006, discloses a method of removing ionic substances from a pulp to be used in the production of electrically insulating paper having a superior dielectric dissipation factor. Although the Fujita et al. process uses an electric field to remove certain substances from a pulp, it was not designed for the deinking of waste paper, the main thrust of it being for the removal of some metallic ions from paper sheets. To achieve its purpose, the Fujita et al. method uses large area electrodes and minimal spacing between electrodes, leading to high maintenance and replacement costs due to the inherent corrosion of the electrodes and the natural migration of foreign particles to the electrode surfaces. Additionally, the Fujita et al. process does not appear to be a retrofit useful on existing deinking equipment.